Gas filling for grid control gas tubes



ep 1951 D. v. EDWARDS ET AL 2,567,369

GAS FILLING FOR GRID CONTROL GAS TUBES Filed Aug. 2, 1947 ZSnventors DVEdwar'ds 4nd EKSm-ith Their (Ittotneg Patented Sept. 11, 1951 GAS FILLING FOR GRID CONTROL GAS TUBES Donald V. Edwards, Montclair, and Earl K. Smith,

West Orange, N. J., assignors to Electrons, Incorporated, Newark, N. J., a corporation of Delaware Application August 2, 1947, Serial No. 765,764

Claims.

This invention relates to electron discharge tubes of the grid controlled gaseous discharge type, commonly termed gas tubes, and more particularly to the structure and the gasfilling for such tubes which will give a short time of effective deionization.

In various applications of grid control gas tubes, the time required for effective de-ionization of the gas filling of the tube after a discharge current has been conducted, suflicient for the control element or grid to regain control, becomes an important factor. For example, when a grid control gas tube is operated in inverters, various types of control equipment and the like, at the higher frequencies, or under conditions where the time available for de-ionization is extremely short, the effective de-ionization time required by the tube imposes restrictions upon the operating frequency that can be employed consistent with reliable performance. The use of higher frequencies is advantageou in many situations, due to the reduction in size and weight of the equipment and other factors; and consequently it is highly desirable to reduce the effective de-ionization time for a grid control gas tube, without affecting its other desirable operating characteristics.

The primary object of this invention is to provide a. gas tube of the grid control type which has a short effective de-ionization time as compared with tubes of alike type and operating characteristics.

Various characteristic features, attributes and advantages of the invention will be in part apparent, and in part pointed out as the description progresses.

General stated, and without attempting to define the nature and scope of the invention, it is proposed to obtain the desired shorter time of effective de-ionization by employing a mixture of certain gases. for the gas filling of a gas. tube, which have certain characteristics in the way of metastable states and. ionizing potentials such that these gases will cooperate or interact in such a way as to reduce the effective de-ionization time for a given tube structure as compared with other gas fi11lllgS More specifically, it is proposed to use a mixture of two of the so-called rare gases, such as Xenon and argon, in roughly equal, proportions at a pressure comparable with that which would be used in the same tube if filled with either one of these gases alone.

For the purpose of facilitating an explanation and understanding of the nature of the invention, the accompanying drawing illustrates in a diagram-matic manner one typical tube structure suitable for use with the gas filling characteristic of this invention, although it should be understood that the same gas filling may be advantageously employed in tubes varying materially in their structural organization, provided certain basic relationships of the tube elements suitable for this type of gas filling are maintained.

In the type of tube illustrated, the. cathode C is of the filamentary type and comprises wires or strips 5, preferably of nickel as a core material, which are formed in a double interlocked spiral in any suitable manner to have the appropriate resistance and emissive area for the heating current and current rating of the tube. These wires or strips 5 are provided with an emissive coating, preferably of the barium oxide type formed and treated in the manner disclosed in our prior Patent No. 1,985,855, December 25, 1934. The cathode C is enclosed in a heat shield HS of the usual type, having the usual discharge opening 5 in its upper end. In the arrangement showmthe upper ends of the spirally wound filaments are connected together and connected to the heat shield HS by suitable tabs orconnectors, as indicated at I. The lower ends of the filaments 5 are connected to a suitable supporting rod 3, which passes through an insulator 9 in the bottom of the heat shield HS, and through a suitable gas tight seal in the stem mount or press It, to which a metal or glass envelope E is attached. lhe heat shield HS is supported on the stem iii by a plurality of supporting rods 13, one of which extends through a seal in the stem to provide an external connection. Heating current is supplied to the external connections provided for the heat shield HS and the lower end of the cathode from a suitable heating transformer or the like (not shown) in accordance with usual practice, so that the heating current flowing through the filaments 5 in multiple raises these filaments to the appropriate emitting temperature.

The diameter of the spiral of the filaments 5 and the spacing between this spiral and the walls of the heat shield HS are selected to conform with the tube rating and the mean free path for ionization of the mixture of gases used for the filling, so that electrons emitted from the emissive surface of the filaments 5 may on the average have an unobstructed path to obtain the necessary ionizing potential for the gas pressure used, in accordance with the disclosure of our prior Patent No. 2,065,997, December 29, 1936.

The control element or grid for the typical tube illustrated comprises a circular flanged ring l5 with parallel grid bars IS welded across a central openin therein, said grid bars being preferably spaced in accordance with the disclosure of our prior Patent No. 1905,692, April 25, 1933, and provided with a coating in accordance with the disclosure of our prior Patent No. 2,012,339, August 27, 1935. A cylindrical skirt or shield I1 is welded to the grid ring l5: and the entire grid assembly is supported by a plurality-of supporting rods IS from the tube stem l0, one of these supporting rods being extended through a suitable seal in said stem to provide an external connection for the grid.

The anode in the typical tube structure illustrated comprises a disc 2 0, preferably of tantalum and having a peripheral flange for stiffness. This anode disc 20 is supported by a rod 2| extending through a seal in the top of the envelope E to provide an external connection to the anode.

The dimensions and space relation of the electrodes for this tube are selected to conform with the intended voltage and current requirements for the tube and the pressure of the gas filling, in accordance with the principles and practices disclosed in our prior patents, such as No. 2,068,539, January 19, 1937, and generally familiar to those skilled in the art.

After the tube elements have been mounted and assembled in the envelope E, the tube is subjected to a degassing and exhaust schedule or procedure in accordance with the practice commonly used for tubes of this type, so that all of the air and other gases, together with the occluded gases in the tube elements and walls of the envelope, are removed, and also the cathode coating is activated. In this connection with this exhaust procedure, the gas filling characteristic of this-invention is introduced into the tube envelope E at the appropriate pressure, and the usual exhaust tube is sealed ofi.

In a typical gas tube structure such as shown and described, an appreciable time must elapse after the anode voltage has fallen below the level required to sustain a discharge, before the gas filling of the tube becomes sufficiently de-ionized for the grid to regain control and prevent initiation of a discharge when the anode voltage again becomes positive. This may be termed the effective de-ionization time for the tube, and imposes limitations upon its application and use. For example, when the tube is operated with anode voltages of the higher frequencies, there ma not be suflicient time for effective de-ionization of the tube between successive positive half cycles of the anode voltage, and the grid is unable to regain control once a discharge through the tube has been initiated. Hence, it is desirable for certain applications of grid control gas tubes to shorten the effective (lo-ionization time for the tube, so

that a given tube having the space relationship of parts, gas pressure and the like suitable for the desired operating characteristics of the tube in respects other than its de-ionization time, may

be operated with anode voltages of the higher frequencies, or in other situations requiring a short effective de-ionization time.

In order that a gas tube may have an acceptable low arc drop, temperature free control characteristics, and other desirable characteristics, it is expedient to use a gas filling of a rare gas, such as xenon, argon or krypton, employing of course a structural organization and spaced relationship of tube elements, together with a gas pressure, suitable for the rating and operating characteristics of the tube desired. With regard to the effective de-ionization time of such a tube, we have found by experimentation and life testing that if one of these gases alone, such as xenon or argon, is used in the tube at the appropriate pressure, the time for effective de-ionization of the tube imposes a certain limit on the frequency of the anode voltage that maybe used and obtain consistent performance, such as for instance in the order of 1500 cycles per second. However, if

two of these gases, such as xenon and argon, are combined in a mixture to form a gas filling at the same pressure in the same tube, repeated tests have shown that the time for effective deionization is cut roughly in half, and reliable tube performance may be obtained with anode voltages at frequencies roughly twice as much as can be used when either of these gases alone is used. Our investigations so far .do not indicate that any critical relative proportions of the two gases of the mixture is necessary; and subject to the limitations of the arc drop desired, and for ordinary operating conditions, we prefer to use as a gas filling a mixture of argon with xenon, or krypton with xenon, in a mixture having approximately 60 to 50 per cent of argon or krypton. Apparently the time of effective de-ionization is somewhat shortened by using a larger proportion of the argon or krypton gas having the higher ionization potential, with a corresponding increase in the arc drop voltage; but this appears to be a matter of degree, rather than a critical proportion; and for general purposes it is contemplated in accordance with this invention that the gas mixture will have roughly equal proportions of the two gases.

We have found as a physical fact that the effective de-ionization time of a given tube sLructure operating at a given gas pressure is greatly reduced by using a mixture of two of the rare gases desirable for the gas filling, rather than only one of these gases alone. The phenomena involved in the de-ionization of the gas filling of a tube after discharge are complex and not readily observable; and we do not have sufiicient acceptable data to offer any definite and substantiated theory in explanation of this established physical fact. Some general observations, however, based on some established theories for electrical discharge of gases, may be helpful in explaining why a gas mixture in accordance with this invention serves to reduce the effective deionization time for a given tube structure.

When the filling of a gas tube is ionized to sustain a discharge current, and positive ions are formed in the plasma, failure of the grid to regain control is usually explained on the theory of the formation of a positive ion sheath around the grid bars, and the time for effective de-ionization is attributed to the time required for deionization of positive ions by recombination at the surfaces of the electrodes or envelope to modify the ion sheath at the grid bars. Our investigations show, however, that the nature of the gas filling is also a factor in this matter of effective de-ionization time, since this time may be greatly reduced for the same tube, presumably having the same period for de-ionization of positive ions, by employing a mixture of two gases, rather than one gas alone. We suggest by way of explanation that this result may be attributed to the different excited and metastable states which the gas molecules, or gas atoms in the case of the monatomic gases under consideration, may have during and immediately following a discharge through the tube.

In explanation of these effects, which are offered merely as a tentative theory of action in connection with this invention, it is generally accepted that upon ionization of the gas filling of a tube during discharge, in addition to electron collisions which completely ionize a gas atom to give a free electron and a positive ion, there are many other electron collisions which may transfor to a gas atom a discrete amount of energy somewhat less than required to separate an electron entirely from the atom and leave an ion, but sufiicient to cause a movement or displacement of an electron from an orbit around the nucleus to some outer orbit, thereby creating what is ordinarily termed an excited state of the gas atom, representing some energy level above its normal state. It has been determined that most gases, and in particular the monatomic rare gases, have certain excited states, commonly known as metastable states, which have a relatively long life span, in the order of 0.1 second. It appears that the lower energy levels for the rare gases, such as xenon for example, correspond closely to these metastable states, so that in a discharge through an xenon gas filling in a tube, there is a marked probability that a substantial number of xenon atoms assume metastable states during discharge.

It is believed that the existence of the metastable state of gas molecules or atoms in a grid control gas tube is a significant factor with respect to the effective de-ionization time of this tube. For one thing, a gas atom in its metastable state is partially ionized so to speak, and may be completely ionized by collision with an electron moving at some speed considerably less than required to ionize a normal atom. Consequently, if there is a sufilcient number of atoms in the metastable state existing at the time the grid of the tube is supposed to regain control, such metastables may be ionized by relatively slow moving electrons existing'in spite of the grid controlling potential, thereby initiating a cumulative ionization independently of the influence of the grid. Also, a gas atom in its metastable state has no electrical charge, and in its random movement during its relatively long life span, may have a collision with some surface of the tube structure, and in giving up its excess energy cause the release of an electron. Such released electrons may then be accelerated by potentials existing between the electrodes and start a reaction of ionization that will cause a discharge in spite of the grid control. For these reasons, it appears to be important in obtaining a short efiective de-ionization time for a gas tube to have a limited number of gas atoms in the metastable state existing within a limited time after the discharge has ceased, so that the control of the grid may become efiective more quickly.

It has been established that gas atoms as a rule have several excited states, corresponding to different energy levels; and in certain of these excited states the gas atom may readily return to its normal state by giving up its excess energy by radiating a light photon. These excited states of gas atoms have very short life spans, in the order of second, and end with the spontaneous return of the excited atom to its normal state, accompanied by the release of a photon. Incidentally, it is this release of photons from the gas atoms in such excited states which accounts for the light associated with the discharge of gas tubes.

Considering the rare gasessuitable for use in gas tubes, it appears to be established by various investigations that the atoms of these rare gases may assume either an excited state having a very short life span terminated by radiation of light giving certain spectra lines, or a metastable state which persists for a comparatively long time until its excess energy is given up in some way by collision with some surface, or like action. As typical of such findings, it has been reported that xenon, with an ionizing potential of 12.08 volts, has a metastable state of 8.28 and an excited state of 9.52. Similarly, krypton with a higher ioniszation potential of 13.94, has a metastable state of 9.86 and an excited state of 9.98; and argon has an ionizing potential of 15.69, a metastable state of 11.49, and an excited state of 11.56.

These different metastable and excited states of these gas atoms, as well as complete ionization, are assumed to be due to the absorption of energy as a result of a collision with a moving electron, or any other particle capable of transferring energy to the gas atom. While the probability of an excited or metastable state of a gas atom resulting from such a collision, as distinctive from the probability of complete ionization, does not appear to have been fully investigated and substantiated, there are persuasive indications that the probability of excitation of a gas atom is the maximum when the energy level of the moving electron or other particle is equal to the excitation potential, decreasing rather rapidly as this energy level exceeds the excitation potential. In other-words, for the purposes of discussion it may be assumed that the probability of excitation of a rare-gas atom to a given excited or metastable state is the greatest when the energy level of the moving electron or other particle is nearest like the excitation potential for such excited state.

Taking into consideration these various phenomena. and theories generally recognized as applicable to discharges in gaseous mediums, the effect of a mixture of two rare gases as proposed in accordance with this invention upon the effective de-ionization of the gas tube, as compared with one gas alone, may be attributed to the more rapid elimination of dissipation of the metastable states of the gasatoms existing at the end of discharge, so that the grid may regain control more quickly, in spite of the existence of gas atoms in the metastable state at the time the discharge ceases. speculating on the probable actions in this respect, during the discharge period the arc drop through the tube largely detBlmlllBS the electron velocity and the energy that may be imparted to the gas atoms by electron collision. This are drop for a typical xenon filled tube varies somewhat, but in tubes with which we are familiar has a general average around 8 volts. It appears that a low energy level for xenon, which is also a metastable state, is about 8.3 volts, so that there is a marked'probability that a substantial. number of xenon atoms will assume metastable states during the discharge. If, however, a mixture of xenon and another rare gas with a higher ionizing potential, such as argon, is used in the tube, the presence of argon with its higher ionization potential raises the arc drop through the same tube to be in the order of 11 to 12 volts; and since this corresponds more nearly to the energy level of about 11.7 for the argon metastable than 8.3 for the xenon metastable, it is believed that argon metastables are more likely to be created during the discharge through this mixture of gases than a xenon metastable, in accordance with the general principle of probability for excited states.

Considering now the critical period when the discharge ceases, if the tube is filled with Xenon alone, the existing xenon metastables persist until in their random motion they may give up their excess energy to some surface of an electrode tube envelope. If however, the tube is filled with a mixture of argon with xenon, thegargon metastables existing when the discharge ceases may in their random motion collide with a xenon gas atom, as well as with electrode surfaces; and in view of the gas density, there is a marked probability that such a collision will occur within a very short time and with a limited amount of random movement of the argon metastable. In the event of such collision, the argon metastable may give up its excess energy to the neutral xenon gas atom, since there is an excited state of the xenon atom having a lower energy level than the argon metastable. Such collision may result in wiping out the argon metastable and creating an excited state of xenon atom, which in turn may quickly revert to its normal state by radiating a photon.

In other words, with a mixture of the two gases xenon and argon, there is an opportunity for collisions between the metastables of argon existing at the time discharge ceases with the xenon atoms also present, which will bring about such exchange of energy that gas atoms in partially excited states capable of interfering with the control function of the grid, are wiped out much more quickly than for either of the gases alone. In this connection, it is more probable that a collision between an argon metastable and -a xenon neutral atom will create an excited state of the xenon atom having a short life span, rather than a metastable state, because the excited state for xenon has an energy level (9.52) more nearly like the energy level (11.49) of the argon metastable than the metastable state (8.28) of xenon.

The action or process thus described to explain why a shorter effective de-ionization time is obtained by a mixture of two rare gases in accordance with this invention, is of the same character and analogous to process of ionization of an argon atom (ionization potential of 15.69) by a metastable neon atom (energy level of 16.53) which has been observed as occurring in a mixture of argon and neon, and also the process of ionization by metastables that has been observed in mercury and argon mixtures.

Irrespective of the accuracy or completeness of this discussion, which is submitted merely for explanatory purposes, the present invention is based upon the demonstrated physical fact that, regardless of the theory of action or cooperation, a mixture in approximately equal proportions of two rare gases suitable for a grid control gas tube, such as xenon with krypton, or xenon with argon, will in fact afford a much shorter time of effective de-ionization at the same pressure and in the same tube structure, than either gas alone will give; and we are claiming as our invention a gas tube including such a mixture of gases, rather than any theory of operation.

From the foregoing it can be seen that the mixture of rare gases in accordance with this invention for the filling of a grid control gas tube enables the effective de-ionization time for such a tube to be greatly reduced, roughly by half, without materially affecting the other desirable operating characteristics of the tube in the way of its grid control ratio, peak forward and inverse voltages, and the like, which are primarily dependent upon the tube structure and gas pressure used, and without objectionable increase in the arc drop voltage for the tube. Such reduction in the effective de-ionization time is of substantial advantage in connection with various applications of such tubes for anode voltages of higher frequencies and the like.

Having pointed out the characteristic features, attributes and advantages of the invention, what we claim is:

1. A grid control gas tube of the character described comprising in combination, a thermionic emissive cathode, a heat shield surrounding said cathode and having a discharge opening therein, a control grid and an anode disposed opposite said opening in the heat shield, and an envelope including these tube elements, said envelope being filled with a mixture in approximately equal proportions of two rare gases having different ionizing potentials and energy levels for metastable states, said gases cooperating to give an effective de-ionization time for the tube much shorter than when filled at the same pressure with either gas alone.

2. An electron discharge tube of the character described comprising in combination, an envelope, a thermionic emissive cathode, control grid and anode included in said envelope, said envelope being filled with a mixture of approximately equal proportions of xenon and argon.

3. A gas tube of the type described comprising in combination, a thermionic emissive cathode, a heat shield surrounding said cathode and having a discharge opening therein, a control grid and anode disposed in a predetermined space relation to said heat shield in accordance with the dimensions of said discharge opening and the space relations of said cathode and heat shield, and an envelope inclosing said tube elements and filled with a mixture of xenon and argon in approximately equal proportions at a pressure comparable with that for which the tube elements are proportioned for xenon gas alone.

4. A grid control gas tube of the character described in combination, a heat shielded thermionic emissive cathode, an anode and a control grid, an envelope enclosing these tube elements in a predetermined space relationship, said envelope being filled with a mixture of xenon and a substantial quantity of another rare gas having a higher ionizing potential, said gases in the mixture cooperating to provide an effective de-ionization time for the tube much shorter than for a filling of either gas alone at the same pressure.

5. A grid control gas tube of the character described comprising in combination, an envelope, a heat shielded thermionic emissive cathode, a control grid and said anode included in an envelope, said envelope being filled with a mixture of xenon and another rare gas in comparable proportions, said other gas having an ionization potential and energy levels for excited and metastable states of its atoms higher than those for xenon, whereby said mixture of gases cooperates to provide a much shorter time for effective de-ionization of the tube than when filled at the same pressure with xenon alone.

DONALD V. EDWARDS.

EARL K. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,316,967 Moore Sept. 23, 1919 1,949,617 Michelssen Mar. 6, 1934 2,116,677 Foulke May 10, 1938 2,159,255 Clark May 23, 1939 2,203,452 Berghaus et a1 June 4, 1940 2,411,241 Arnott et al Nov. 19, 1946 

